Visualforce Remoting with Relationships

remoting.page

<apex:page controller="RemotingController">

<script type="text/javascript">

function Test_Object__c() {
  this.Name = '';
  this.Test_Object_2__r = null;              
}

function Test_Object_2__c() {
  this.Name = '';
}

var b = new Test_Object_2__c();
b.Name = 'New B';

var a = new Test_Object__c();
a.Name = 'New A';
a.Test_Object_2__r = b;

Visualforce.remoting.Manager.invokeAction(
  '{!$RemoteAction.RemotingController.UpsertA}',
  a,
  function(result, event) {
    console.log(result);
  }
);
</script>

</apex:page>

RemotingController.cls

global class RemotingController {

// Assumes two custom objects: Test_Object__c with lookup to Test_Object_2__c
    @RemoteAction
    global static void UpsertA(Test_Object__c obj1) {
      upsert obj1.Test_Object_2__r;
      obj1.Test_Object_2__c = obj1.Test_Object_2__r.Id;
      upsert obj1;
    }
    
}